Multicarrier systems hold great potential to provide high bit-rates in multipath fading channels. The ability of such systems to robustly handle multipath and their dramatically simple equalizer structures have made multicarrier modulation a preferred choice in contemporary communication systems. Already two strong contenders for 4G systems are based on multicarrier modulation: MIMO-OFDM and MC-CDMA. Even recent generations of other technologies have relied heavily on multicarrier signals, including OFDM as specified in the United States under IEEE 802.11a. Similar multicarrier regimens are specified in Japan by the ARIB MMAC group, and in Europe by the ETSI workgroup BRAN as HIPERLAN, each operating in the 5 GHz range and enabling user-selectable data rates of up to 54 Mbits/sec. A conceptual diagram of an OFDM system according to 802.11a is shown in FIG. 1. Between the frequencies of 5150 and 5350 MHz are eight non-overlapping orthogonal channels (for the two lower bands). Each of those channels is divided into 52 subcarriers, each occupying 312.5 kHz of bandwidth. Four of those subcarriers carry “pilot tones” that do not carry transmit data but are instead used to aid in coherent demodulation of the signal at the receiver, leaving 48 subcarriers to carry the user information. Whether OFDM or MC-CDMA, the underlying theme for multicarrier communication is usually centered around synchronous addition of several subcarriers. The signal obtained by coherent addition of several subcarriers (hence, multicarrier) is transmitted such that the receiver can demodulate individual subcarriers rather easily. The ease of demodulation is ensured either by inherent orthogonality of the subcarriers (where a subcarrier might represent an individual user) or by orthogonal Walsh codes, such as used in CDMA.
An important goal of multicarrier system research is to devise methods to efficiently handle signals with a large dynamic range. It is common to quantify the signal's dynamic range in terms of the peak to average power ratio (PAR). Typically, a high PAR results from the occasional (but unpredictable) coherent addition of subcarriers leading to instantaneous peaks. Moderate power levels may be readily handled by the RF power amplifier (PA) of the prior art, but the occasional peak powers pose severe problems. A high input signal PAR forces the RF power amplifier (RFPA) to operate at several dBs of output power backoff, leading to undesirably low efficiencies. The PAR problem is generally more pronounced in multi-carrier systems because it is directly impacted by the bandwidth of the system. For example, the PAR in an OFDM system is proportional to the number of subcarriers N, and larger numbers of subcarriers N (for a given subcarrier spacing), result in larger system bandwidth. The PAR problem appears to be inherent to all multicarrier modulated systems such as OFDM because the multiple subcarriers can add together constructively to create a very large signal, or destructively to create a very small signal. The wide variation makes for a challenging power amplifier design, as distortion must be minimized while keeping the average power low enough to accommodate the large peaks.
A reduced PAR and higher power efficiency is especially desirable in mobile devices because it would directly reduce power requirements. This could result in a significant reduction in power consumption, an important consideration for portable devices that rely on batter power. It may also enhance the user experience where reduced power consumption yields a noticeable reduction in heat dissipation through the handheld device. The traditional approach to RFPA design for managing high PAR has been to innovate single-transistor amplifiers with progressively higher power rating, which generally results in further challenges to maintain amplifier linearity.
Regardless of the specific approach, prior art efforts to reduce PAR generally come at a cost of increased bandwidth or lower data throughput. One common method used in the prior art to reduce PAR is to clip the signal any time the envelope amplitude exceeds a clipping threshold. This technique carries two disadvantages: signal fidelity is reduced because signal energy is discarded each time the peak-valued signal samples are clipped; and clipping is necessarily an amplitude compression technique that leads to bandwidth expansion in the frequency domain (though the expansion is subtle). Additionally, the effectiveness of clipping is inversely proportional to the order of modulation, so clipping a 64-QAM signal is less effective than clipping a 16-QAM signal.
Companding, which generally comprises amplitude compression followed by expansion, is another method to reduce system PAR. Even more so than clipping, companding improves the PAR at the expense of bandwidth. Because companding results in much higher expansion of bandwidth as compared to clipping techniques, it is not very amenable to systems using multipath transmission channels due to the increased bandwidth that multicarrier systems employ, as noted above. Other attempts to reduce PAR include using block codes to modulate the subcarriers rather than using the data directly, and to restrict the subcarrier modulation schemes to some defined phase and amplitude relationship. These approaches generally result in a bandwidth cost. For example, in the case of coding methods, the increased bandwidth cost is exhibited as a reduction in coding rate for the system. What is needed in the art is a method and apparatus to reduce PAR in a multicarrier system without increasing bandwidth or losing data throughput.